FD solver confusion (on my part)

[Conrad Scott]

Dear users-prolog list,

I've just started using GNU Prolog, in particular its FD solver 
sub-system, so I'm not an expert at either and this is my first post to 
the list. I'm a bit confused about the way the constraints solver 
works. For example, if I make the following query I get what I expect:

        | ?- A#=B,fd_domain([A,B],0,1),fd_labeling([A,B]).

        A = 0
        B = 0 ? a

        A = 1
        B = 1

        yes

But if I re-order the query as follows I'm a little surprised at the 
result:

        | ?- fd_domain([A,B],0,1),A#=B,fd_labeling([A,B]).

        A = 0
        B = 0 ? a

        A = 1
        B = 0

        yes

I assume that this is a problem of my understanding (rather than a 
bug), so can someone point out why the results differ (or point me to 
some applicable documentation, by preference in English, but in French 
if that's the best information).

Thanks in advance,

Conrad